1. Field of the Invention
The present invention relates to an automatic gain control method, and more specifically, to an automatic gain control method capable of detecting transmission mode and perform gain control in a communication system having a discontinuous transmission mode.
2. Description of the Prior Art
In frame based wireless communication systems, a typical transmission method is to interleave data into a plurality of frames and organize the frames to form a plurality of signal blocks. For a receiver to receive the signal transmitted by a transmitter correctly, the transmitter and receiver must modulate and demodulate synchronously.
Please refer to FIG. 1. FIG. 1 is a schematic diagram illustrating a 104-frame structure (B0-B23) of the GSM (Global System for Mobile Communication) system. In FIG. 1, the frames marked S denotes the SACCH control frames and the frames marked I denotes the Idle frames. In the digital wireless communication system according to the prior art, when the transmitter is in a discontinuous transmission mode (DTX mode), the automatic gain control unit in the receiver can use at most 12 specific frames to perform gain control. These 12 specific frames include 4 SACCH (Slow-Associated Control Channel control frames that are between the blocks B5 and B6, the blocks B11 and B12, the blocks B17 and B18, and the blocks B23 and the block B0 of the next 104-frame structure, and 8 SID frames (Silence Information Description Frame) that are between the block B12 and B13. In addition, the gain estimated according to the SACCH frame between the blocks B5 and B6 is used as the initial gain value for receiving the blocks B6 to B11. The gain estimated according to the SACCH frames between the blocks B11 and B12 and between the blocks B12 and B13 is used as the initial gain value for receiving the blocks from B14 to B17. The gain estimated according to the SACCH frame between the blocks B17 and B18 is used as the initial gain value for receiving the blocks B18 to B23. In addition, the gain estimated according to the SACCH frame between the blocks B23 and BO in the next 104-frame structure is used as the initial gain value for receiving the blocks B0 to B5 in the next 104-frame structure.
As shown in FIG. 1, in the digital wireless communication system according to the prior art, the gain controlling process depends only on 4 SACCH control frames and 8 SID frames no matter the transmission is in the DTX mode or not. In practical, the number of the frames utilized by the prior art is too small. That is the sample space for the gain control algorithm is too small such that the gain tracking speed is slow and the tracking accuracy is poor. It is desired to have an automatic gain control method which is capable of utilize those frames as much as possible to enlarge the sample space and achieve a better gain control.